Magnetic resonance imaging with high spatial and temporal resolution

ABSTRACT

Methods, systems, and apparatus for magnetic resonance imaging (MRI) are described. In one example, the method includes applying a sample-selective magnetic field gradient sequence along a slice direction to partition responses from a sample under magnetic resonance imaging into a plurality of different regions of slices in the k space, applying spiral readout gradients to the sample to obtain echo data from the sample, and applying a Time Resolved Imaging of Contrast Kinetics imaging technique to sample the different regions of slices in the k space.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent application Ser. No. 11/735,415, filed Apr. 13, 2007, which claims the benefit of the filing date of U.S. Provisional Application No. 60/792,026, filed Apr. 13, 2006, and entitled “High Resolution Time-Resolved Contrast-Enhanced Magnetic Resonance Angiography Using Spiral-TRICKS Sequence,”. The entire contents of the before-mentioned patent applications are incorporated by reference as part of the disclosure of this application.

TECHNICAL FIELD

This application relates to magnetic resonance imaging (MRI) and magnetic resonance angiography.

BACKGROUND

Imaging through MRI techniques can be applied in imaging applications in medical, biological, and other fields. An MRI technique can produce an image of a selected part of an object under examination by manipulating the magnetic spins in the part and processing measured responses from the magnetic spins. Three-dimensional (3D) contrast-enhanced magnetic resonance angiography (CE-MRA) can be used for the evaluation of vasculature of most organs in humans. Conventional CE-MRA examinations can be performed using a single phase 3D Fourier Transform gradient echo acquisition of 20-30 seconds.

Time Resolved Imaging of Contrast Kinetics (TRICKS) is a 3D imaging technique developed by Frank R. Korosec, et. al at the University of Wisconsin, Madison, and commercialized by General Electric (GE) Healthcare for rapid dynamic imaging. TRICKS can offer increased temporal resolution compared to conventional dynamic sequences. In one implementation, the TRICKS sequence can divide 3D Cartesian k-space into several subvolumes (for example, regions A, B and C) located at increasing distance from the k-space center. The k-space is an extension of the concept of Fourier space. TRICKS can reacquire k-space center (region A) more often than outer sections (regions B and C). One example of a TRICKS acquisition scheme is: ABACABACA . . . TRICKS can be combined with other imaging techniques including under-sampled projection reconstruction (PR).

SUMMARY

In one example, implementations of a fast dynamic imaging sequence called Spiral-TRICKS is described. The implementations can employ spiral readout in-plane and Cartesian volume encoding. The combination of spiral trajectory and TRICKS acquisition can provide time-resolved 3D images with high temporal resolution, high spatial resolution and high signal-to-noise ratio (SNR).

In one aspect, a method for magnetic resonance imaging (MRI) includes applying a sample-selective magnetic field gradient sequence along a slice direction to partition responses from a sample under magnetic resonance imaging into a plurality of different regions of slice is in the k space, applying spiral readout gradients to the sample to obtain echo data from the sample, and applying a Time Resolved Imaging of Contrast Kinetics imaging technique to sample the different regions of slices in the k space.

This, and other aspects, can include one or more of the following features. The method can include controlling a delay in a gradient in the MRI to reduce a distortion and anisotropy in the echo data from the sample. The method can include measuring a phase difference in MRI data of the sample with and without readout gradients, mapping each spiral trajectory in the k space using the measured phase difference, and obtaining k space trajectory measurements using 2D off-center excitation to reduce any eddy current effect in the MRI image. The method can further include using field map measurements by two single short spiral acquisitions at two different echo times to reduce and off-resonance effect in the MRI image. The method can further include measuring k-space trajectories for two low-resolution acquisitions, performing image regridding and fast Fourier transform (FFT) on the k space trajectories for two low resolution acquisitions to generate to complex images, respectively, and using a phase difference between the two complex images to provide an off-resonance map for correcting a phase error.

In another aspect a method for magnetic resonance imaging (MRI) of a sample can include obtaining sample information in a spiral readout trajectory in-plane in a k-space from a section of the sample, employing volume encoding along a slice direction in the k-space in the region, wherein the slice direction is orthogonal to the in-plane, partitioning the encoded section into a plurality of regions in the k-space, the plurality of regions including a central region at the k-space center, and acquiring sample information from the plurality of regions, wherein the sample information from the central region is acquired more often than the sample information from regions away from the k-space center.

This, and other aspects, can include one or more of the following features. The method can include partitioning the encoded section into a plurality of regions of the same size in the k-space. The method can further include assigning regions near the k-space center as little frequency regions. The method can further include assigning the regions away from the k-space center as high frequency regions. The method can further include dividing spiral trajectories substantially in the central region into a plurality of subsets of interleaves. The method can further include covering one subset of interleaves with sample information acquired from the central region. The method can further include acquiring sample information by asymmetric k-space sampling. Acquiring sample information by the asymmetric k-space sampling can include acquiring sample information from all of the central region and portions of non-central regions. Sample information can be acquired using a Time Resolved Imaging of Contrast Kinetics (TRICKS) imaging technique.

In other aspects, computer program products, embodied on a computer readable media, operable to cause a data processing apparatus to perform the methods described above is described.

In other aspects, systems for magnetic resonance imaging (MRI) are described. The systems include a magnetic field source configured to generate a magnetic field, a radio frequency system configured to transmit and receive radio frequency signals, a sample holder configured to receive and hold a sample, and a data processing apparatus configured to perform the methods described above.

The systems and techniques described here can be implemented in ways that achieve one or more of the following advantages in various applications. The described techniques can be used to achieve contrast-enhanced magnetic resonance angiography which exhibits complete contrast dynamics with excellent separation between the arterial and venous vascular systems. The 3D stack of spiral sequence can be used to provide higher efficiency than some other 3D Cartesian sequences or hybrid projection reconstruction sequences. In addition, the spiral trajectory described in this application can be used to allow longer sampling intervals in MRI imaging and offer more efficient sampling of raw data by circular coverage by the trajectory in the k-space. The complete k-space can be covered with fewer excitations without under-sampling artifact. Furthermore, spiral trajectories can permit sampling to begin at the k-space center, which can significantly reduce echo time (TE) and provide intrinsic flow compensation. The combination of spiral and TRICKS acquisition can significantly increase the acquisition speed while preserving high spatial resolution and SNR.

These and other implementations are set forth in the accompanying drawings, the description, and the claims. Other features and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic of k-space sampling using a 3D stack of spirals trajectory.

FIG. 1B is a schematic of interleaved spiral trajectory for in-plane sampling.

FIG. 1C is a schematic of data acquisition.

FIG. 2 is a plot of magnitude and phase for trajectory measurements.

FIG. 3A is a plot of measured and ideal spiral trajectory.

FIG. 3B is a plot of k-space deviation of measured spiral trajectory.

FIGS. 4A, 4B, and 4C are phantom images with and without minimization of distortions.

FIG. 5 is the complete contrast dynamics of the pulmonary vasculature.

FIG. 6 are 3D pulmonary images shown from different angles.

FIG. 7 are series of images depicting the complete contrast dynamics in the renal arteries.

FIG. 8 are rotation of two consecutive time frames to display spatial resolution and fast enhancement.

FIG. 9 is a flow chart of an example of a process of magnetic resonance imaging.

FIG. 10 is a flow chart of an example of a process of magnetic resonance imaging.

FIG. 11 is an example of an MRI system.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1A depicts a k-space 100 sampling using a 3D stack of spirals trajectory with kZ slices 105 partitioned into four regions: A, B, C, and D. In some implementations, a sample-selective magnetic field gradient sequence is applied to a sample along a slice direction to partition responses from the sample under magnetic resonance imaging into a plurality of different regions of slices in the k space. Applying such a magnetic field to sample can enable volume-encoding in MRI measurements. The Spiral-TRICKS sequence can use the spiral readout trajectory in the XY plane, and the Fourier encoding along the slice direction, e.g., the Z direction, in k-space 100. The spiral trajectory can allow for a much longer data acquisition window with an arbitrary imaging field of view (FOV) to achieve a high spatial resolution. The Spiral-TRICKS sequence can combine the high SNR efficient spiral trajectory 110 with high acquisition efficient TRICKS technique for fast dynamic imaging.

In the Spiral-TRICKS sequence, the volume encodings can be partitioned into multiple regions (A, B, C, and D) from the low frequency encoding to high frequency encoding. The partitioned k-space 100 regions can be sampled through TRICKS acquisition order (A₁B₂A₃C₄A₅D₆A₇B₈ A₉C₁₀A₁₁D₁₂A₁₃B₁₄ . . . ), where the subscript can refer to the acquisition order. Regions A, B, and C can typically be of the same size and can contain the same number of slices. Region D can be the same size of other regions or greater in size than other regions, e.g., three times the size of other regions.

FIG. 1B depicts interleaved spiral trajectory used for in-plane sampling. The spiral trajectories 110 can be interleaved in k-space 100. To further speed up the acquisition, spiral trajectories 110 from region A can be divided into three subsets of interleaves and each region A acquisition can cover one subset spiral interleaves. In the slice direction, asymmetric k-space sampling, or partial encoding, can be employed to further shorten the total acquisition time. Slices from region A can be fully sampled, while slices from regions B, C, and D can be partially sampled. For example, only one half of regions B, C, and D can be sampled. As a result, only 58% of the slice partitions can be acquired. Homodyne reconstruction along the slice direction can be carried out to recover the slice resolution.

FIG. 1C depicts the details of one implementation of the Spiral-TRICKS pulse sequence. In some implementations, mask data can be acquired for all regions followed by dynamic data acquisition in the order used in TRICKS. A sliding window reconstruction can be used to generate high spatial resolution time frame images. As depicted in FIG. 1C, the first part of the data acquisition can be for mask. The subsequent dynamic phase acquisition can cover the complete contrast dynamics, including the early arterial phase, peak arterial phase and venous phase. In the reconstruction, k-space subtraction can be performed to suppress background tissue, which is not enhanced upon contrast agent arrival, therefore increasing the image contrast where the blood vessels are enhanced due to the T1 shortening contrast agent. A sliding window reconstruction scheme can be adapted, where the first time frame image can be reconstructed from A₁B₂A₃C₄A₅D₆, the second time frame image can be reconstructed from B₂A₃C₄A₅D₆A₇, the third time frame image can be reconstructed from A₃C₄A₅D₆A₇B₈, etc.

The effects of gradient distortion, eddy current, and off-resonance can be minimized. In some implementations, the gradient distortion and anisotropy can be minimized through gradient delay calibration (a coarse tuning). The gradient delay along each logical gradient can be changed by a predetermined time unit, e.g., 4 μsec, causing the error to be reduced by a corresponding time unit, e.g., 2 μsec. Furthermore, the data acquisition window can be shifted by a time unit, e.g., 1 μsec. Thus, gradient delay errors can be minimized within 1 μsec.

The eddy current effect can be minimized through the k-space trajectory measurement using 2D off-center excitation. Under the 2D off-center excitation, the phase difference with and without readout gradients can be measured to accurately map the true k-space trajectory. Subsequently, the true k-space trajectory can be used to regenerate the grid of raw image data in the k-space. This process of regenerating the grid of raw image data in the k-space is termed regridding. Regridding is a fine tuning process for the k-space distortion. This step can be performed after the gradient delay calibration.

Off-resonance effects are signals that have a frequency different from that expected for a nucleus in an idealized system with perfectly uniform static magnetic field throughout the sample, and perfect linear gradients. The off-resonance effect can be minimized through field map measurements using two single short spiral acquisitions at two different echo times. The k-space trajectories for the two low resolution acquisitions can also be measured, followed by regridding and Fast Fourier Transform (FFT) to generate two complex images. The phase difference between the two low resolution acquisitions can be used to provide an off-resonance map, which can be used to correct the phase error. The k-space trajectory error can be corrected using the measured trajectory.

FIG. 2 depicts a first plot of magnitude of spiral calibration raw data with gradient on and gradient off measured across spiral readout and a second plot of phase for spiral calibration raw data with gradient on and gradient off measured across spiral readout. The magnitude and phase for trajectory measurements can be measured using 2D off-center slice excitation with readout gradient on and off. The phase difference between these two acquisition can be used for true trajectory mapping.

FIG. 3A depicts measured and ideal spiral trajectory for the first 100 out of 512 points. FIG. 3B depicts k-space deviation of measured spiral trajectory for Gx gradient and Gy gradient. FIGS. 2-4 show more information regarding to spiral trajectory measurement and phantom images with and without the above three-step correction algorithm.

FIGS. 4A, 4B, and 4C depict phantom images with and without corrections by minimization of the gradient distortion, eddy current, and off-resonance. FIG. 4A depicts phantom images without any correction. The correction can be the minimizations of gradient distortion, eddy current, and off-resonance. FIG. 4B depicts phantom images with k-space trajectory measurement. FIG. 4C depicts k-space trajectory measurements plus B0 field map correction. B0 field map can correct the phase errors. The k-space distortion can be corrected using spiral trajectory measurement. Eddy current and gradient distortion can be reduced after trajectory measurement. B0 field map correction can help for signal recovery at the periphery of the imaging FOV, where B0 non-homogeneity can be significant.

The first spiral-TRICKS volunteer scan was performed for pulmonary MRA using an eight element cardiac phased array coil offered by GE Medical Systems (Milwaukee, Wis.). The acquisition parameters for pulmonary imaging are: FOV=30×30 cm, TR/TE=5.1/0.67 ms, flip angle=30°, BW=125 kHz, 512 points per interleaf, 72 spiral interleaves, 36 kz volume encodings in the anterior/posterior direction, slice thickness=3 mm, recon matrix=512×512. A contrast agent (Gd-BOPTA) of about 30 ml was injected intravenously, with an injection rate of 3.0 ml/sec followed by 20 ml flush at the same injection rate. The total scan time was 30 seconds, including mask. The first 7.5 seconds were for mask, and the remainder for the dynamic imaging.

FIG. 5 shows the coronal maximum intensity projection (MIP) time frames. Complete contrast dynamics of the pulmonary vasculature was depicted through 15 consecutive time frame images with a temporal resolution of about 1.0 second-per-frame. The acquisition FOV of 300×300×10⁸ mm³, spiral readout of 512, and 72 spiral interleaves result in an acquired voxel size of 1.56×1.56×3.0 mm³, which was zero-padded to 0.63×0.63×0.63 mm³ in the reconstruction. Fifteen time frames were displayed with a high temporal resolution of 1.0 seconds-per-frame which captured the complete contrast dynamics in the lung, including arterial phases, arterial venous transition phases, and venous phases. The high in-plane spatial resolution of 1.56 mm, which was zero-padded to 0.63 mm, provided an excellent depiction of the pulmonary vessels. No significant image blurring was observed because of the three-step corrections.

FIG. 6 depicts the 3D pulmonary images shown from three different angles. The blurring along the slice resolution can be seen because the slice thickness is 3 mm, while the in-plane resolution is 1.56 mm. Both coronal and rotated images were shown in FIG. 6, which demonstrated high in-plane spatial resolution and moderate slice resolution, which was 3 mm thick and zero-padded to 0.63.

A renal study was also performed using Spiral-TRICKS sequence. The acquisition parameters for renal imaging are: FOV=30×30 cm, TR/TE=7.4/0.8 ms, flip angle=30°, BW=125 kHz, 1024 points per interleaf, 48 spiral interleaves, 36 kz volume encodings in the anterior/posterior direction, slice thickness=2.0 mm, reconstruction matrix size=512×512. The complete contrast dynamics were well depicted through 15 consecutive time frames with high in-plane spatial resolution of 1.28 mm (zero-padded to 0.63 mm) and through-plane resolution of 2.0 mm (zero-padded to 0.63 mm). The arterial phase and portal venous phase were well separated despite the fast transition.

FIG. 7 depicts fifteen time series of images of the complete contrast dynamics in the renal arteries. The depiction of the arterial phase, arterial-venous transition phase and portal venous phase can be seen. The acquisition of FOV of 300×300×72 mm³, spiral readout of 1024, and 48 spiral interleaves result in an acquired voxel size of 1.28×1.28×2.0 mm³ in the reconstruction. Other imaging parameters include TR/TE=7.4/0.8 ms, flip angle=30°, bandwidth=125 kHz.

FIG. 8 shows the rotation of two consecutive arterial phases, which demonstrated the high spatial resolution and quick renal parenchymal enhancement. A high temporal resolution is required to capture the details of the contrast enhancement.

FIG. 9 is a flow chart of an example of a process for magnetic resonance imaging of a sample. In some implementations, a slice-selective magnetic field gradient sequence can be applied along a slice direction to partition responses from a sample under magnetic resonance imaging. In addition, spiral readout gradients can be applied to the sample to obtain echo data from the sample. Further, a TRICKS imaging technique can be applied to sample the different regions of slices. In some implementations, a delay in a gradient in the MRI can be controlled to reduce a distortion and anisotropy in the echo data from the sample. In some implementations, a phase difference in MRI data of the sample can be measured with and without readout gradients. In addition, each spiral trajectory in the k-space can be mapped using the measured phase difference, and the k-space trajectory measurements can be obtained using 2D off-center excitation to reduce an eddy current effect in the MRI image.

In some implementations, field map measurements can be used by two single short spiral acquisitions at two different echo times to reduce an off-resonance effect in the MRI image. In addition, the k-space trajectories for two low resolution acquisitions can be measured. Image regridding and fast Fourier transform (FFT) can be performed on the k-space trajectories for two low resolution acquisitions to generate two complex images, respectively. A phase difference between the two complex images can be used to provide an off-resonance map for correcting a phase error.

FIG. 10 is a flow chart of an example of a process for magnetic resonance imaging (MRI) of a sample. In some implementations, sample information can be obtained in a spiral readout trajectory in-plane in a k-space from a section of the sample at 1005. A spiral readout trajectory in-plane can be employed in a k-space to a volume encoding of the sample. A Fourier encoding can be employed along a slice direction to the encoding in the k-space at 1010. The slice direction can be orthogonal to the in-plane. The sample encoding can be partitioned into a plurality of regions in the k-space at 1015. The plurality of regions can include a central region at the k-space center. Sample information can be acquired from the plurality of regions at 1020. The sample information from the central region can be acquired more often than the sample information from regions away from the k-space center.

Each region of the plurality of regions can be of the same size. The regions near the k-space center can correspond to low frequency encoding. Regions away from the k-space center can correspond to high frequency encoding. The spiral trajectories substantially in the central region can be divided into a plurality of subsets of interleaves. The spiral trajectories can be located either at the center of the k-space or near the center of the k-space within the central region. The information acquired from the central region can cover one subset of interleaves. Sample information can be acquired by asymmetric k-space sampling. In asymmetric k-space sampling, sample information can be obtained from all of the central region and portions of the non-central regions. In other implementations, the sample information can be acquired by a TRICKS imaging technique.

An MRI system may include hardware to generate different magnetic fields for imaging, including a static magnetic field along a z-direction to polarize the magnetic spins, gradient fields along mutually orthogonal x, y, or z directions to spatially select a body part for imaging, and an RF magnetic field to manipulate the spins. The above spiral-TRICKS sequence may be applied in various imaging applications, including but not limited to, (1) contrast-enhanced MRA of pulmonary, renal, carotid and peripheral vasculature, (2) brain arterial venous malformation (AVM), (3) perfusion imaging based on dynamic contrast enhancement information, and (4) tumor and lesion characterization, including breast tumor, liver lesion, prostate tumor, etc, where the contrast enhancement pattern is an indication of benignity or malignity of the tumor.

FIG. 11 is an example of an MRI system 1100. The MRI system 1100 can be implemented using any one of various MRI scanners such as a 1.5 T Sigma TwinSpeed scanner (available from GE Healthcare Technologies, Milwaukee, Wis.). The MRI system 1100 can include a scanner 1105, a data processing apparatus 1145, and a sample holder or table 1140 for holding a sample 1135. The scanner can include a main magnet 1110, three orthogonal gradient coils 1130 and an RF system 1115. The main magnet 1110 can be designed to provide a constant, homogeneous magnetic field. The three orthogonal gradient coils 1130 can be designed to provide three orthogonal, controller magnetic gradients used to acquire image data of a desired slice by generating an encoded and slice-selective magnetic field. The RF system 1115 can include an RF transmit coil 1120 and an RF receive 1125 designed to transmit and receive RF pulses. The RF system 1115 can further include an RF synthesizer (not shown) and a power amplifier (not shown). In some implementations, an integrated transceiver coil (not shown) can be implemented instead of the separate transmit coil 1120 and receive coil 1125 for transmitting and receiving RF signals. For example, a close-fitting smaller coil can improve image quality when a small region is being imaged. Further, various types of coils that are placed around specific parts of a body (e.g., the head, knee, wrist, and the like) or even internally can be implemented depending on the sample and imaging applications. The MRI system 1100 can generate different magnetic fields for imaging, including a static magnetic field along a z-direction to polarize the magnetic spins, gradient fields along mutually orthogonal x, y, or z directions to spatially select a body part for imaging, and an RF magnetic field to manipulate the spins. The data processing apparatus (e.g., a computer 1145 can be designed to receive and process the data obtained from the sample. In addition, the data processing apparatus 1145 can include instructions to instruct the MRI system 1100 to apply the RF signals that generate a slice-selective magnetic field gradient sequence along a slice direction to the sample 1135. Further, the data processing apparatus 1145 can include instructions to apply spiral readout gradients to the sample to obtain echo data. Furthermore, the data processing apparatus 1145 can apply TRICKS imaging technique to the sample, and any other image processing technique to the sample.

Embodiments of the subject matter and the functional operations described in this specification can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer program products, i.e., one or more modules of computer program instructions encoded on a tangible program carrier for execution by, or to control the operation of, data processing apparatus. The tangible program carrier can be a propagated signal or a computer readable medium. The propagated signal is an artificially generated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus for execution by a computer. The computer readable medium can be a machine-readable storage device, a machine-readable storage substrate, a memory device, a composition of matter effecting a machine-readable propagated signal, or a combination of one or more of them.

The term “data processing apparatus” encompasses all apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them.

A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, or declarative or procedural languages, and it can be deployed in any form, including as a stand alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network.

The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).

Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device.

Computer readable media suitable for storing computer program instructions and data include all forms of non volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry.

To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, input from the user can be received in any form, including acoustic, speech, or tactile input.

Embodiments of the subject matter described in this specification can be implemented in a computing system that includes a back end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation of the subject matter described is this specification, or any combination of one or more such back end, middleware, or front end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), e.g., the Internet.

The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other.

While this specification contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

Only a few examples are described. Other implementations, variations and enhancements may be made based on what is disclosed in this application. 

1. A method for magnetic resonance imaging (MRI) of a sample, comprising: obtaining sample information in a spiral readout trajectory in-plane in a k-space from a section of the sample; employing volume encoding along a slice direction in the k-space in the region, wherein the slice direction is orthogonal to the in-plane; partitioning the encoded section into a plurality of regions in the k-space, the plurality of regions including a central region at the k-space center; and acquiring sample information from the plurality of regions, wherein the sample information from the central region is acquired more often than the sample information from regions away from the k-space center.
 2. The method of claim 1, further comprising partitioning the encoded section into a plurality of regions of the same size in the k-space.
 3. The method of claim 1, further comprising assigning regions near the k-space center as low frequency regions.
 4. The method of claim 3, further comprising assigning the regions away from the k-space center as high frequency regions.
 5. The method of claim 1, further comprising dividing spiral trajectories substantially in the central region into a plurality of subsets of interleaves.
 6. The method of claim 5, further comprising covering one subset of interleaves with sample information acquired from the central region.
 7. The method of claim 1, further comprising acquiring sample information by asymmetric k-space sampling.
 8. The method of claim 7, wherein acquiring sample information by the asymmetric k-space sampling comprises acquiring sample information from all of the central region and portions of non-central regions.
 9. The method of claim 1, wherein sample information is acquired using a Time Resolved Imaging of Contrast Kinetics (TRICKS) imaging technique.
 10. A computer program product, embodied on a non-transitory computer readable medium, operable to cause a data processing apparatus to perform the method of claim
 1. 11. The computer program product of claim 10, wherein each region of the plurality of regions are of the same size.
 12. The computer program product of claim 10, wherein regions near the k-space center correspond to low frequency encoding.
 13. The computer program product of claim 12, wherein regions away from the k-space center correspond to high frequency encoding.
 14. The computer program product of claim 10, the operations further comprising dividing spiral trajectories substantially in the central region into a plurality of subsets of interleaves.
 15. The computer program product of claim 14, wherein sample information acquired from the central region covers one subset of interleaves.
 16. The computer program product of claim 10, the operations further comprising acquiring sample information by asymmetric k-space sampling.
 17. The computer program product of claim 16, wherein acquiring the sample information by the asymmetric k-space sampling comprises acquiring sample information from all of the central region and portions of non-central regions.
 18. The computer program product of claim 10, wherein sample information is acquired using a Time Resolved Imaging of Contrast Kinetics (TRICKS) imaging technique.
 19. A system for magnetic resonance imaging (MRI), comprising: a scanner configured to generate a magnetic field; a radio frequency system configured to transmit and receive radio frequency signals; a sample holder configured to receive a sample; and a data processing apparatus configured to: obtain sample information in a spiral readout trajectory in-plane in a k-space from a section of the sample; employ volume encoding along a slice direction in the k-space in the region, wherein the slice direction is orthogonal to the in-plane; partition the encoded section into a plurality of regions in the k-space, the plurality of regions including a central region at the k-space center; and acquire sample information from the plurality of regions, wherein the sample information from the central region is acquired more often than the sample information from regions away from the k-space center.
 20. The system of claim 19, wherein the data processing apparatus is further configured to partition the slice encoded section into a plurality of regions of the same size in the k-space.
 21. The system of claim 19, wherein the data processing apparatus is further configured to assign regions near the k-space center as low frequency regions.
 22. The system of claim 21, wherein the data processing apparatus is further configured to assign the regions away from the k-space center as high frequency regions.
 23. The system of claim 19, wherein the data processing apparatus is further configured to divide spiral trajectories substantially in the central region into a plurality of subsets of interleaves.
 24. The system of claim 23, wherein the data processing apparatus is further configured to cover one subset of interleaves with sample information acquired from the central region.
 25. The system of claim 19, wherein the data processing apparatus is further configured to acquire sample information by asymmetric k-space sampling.
 26. The system of claim 25, wherein the data processing apparatus is further configured to acquire sample information by the asymmetric k-space sampling from all of the central region and portions of non-central regions.
 27. The system of claim 19, wherein the data processing apparatus is further configured to acquire sample information using a Time Resolved Imaging of Contrast Kinetics (TRICKS) technique. 